Polymerization dioxane-forming

The reaction of/ -dibromobenzene with J-BuLi in dioxane forms / -bromophenyl lithium, which by addition of HMPA polymerizes to polyphenylenes165. The same reaction in THF (-78 °C), after warming to room temperature, also leads to polyphenylenes166. Although there are no mechanistic studies, it was suggested that this polymerization occurs by an S l reaction 166. 

Since most of the synthetically useful enolate anions described in the previous section are prepared by the reactions of enolizable substrates with alkali metal amide bases, it is appropriate to note a few structures of these amide bases. The common bases in synthetic organic chemistry include LDA and LHMDS. The structures of both of these bases are known as the THF solvates.Both of these compounds form bis-solvated dimers corresponding to structure (201). The diethyl ether solvate of LHMDS also forms a bis-solvated dimer (202).Sodium hexamethyldisilazide crystallizes as an unaggregated monomer from benzene solution.Two different cryst line forms of KHMDS are known as the polymeric dioxane solvate (203), ° and the unsolvated dimer (204). ... 

Additional empirical observations concerning this reaction were that use of BF3 as its etherate or addition of small amounts of water (both apparently common practices in certain polymerizations) reduce the overall yield of cyclic products because of chain termination. In a typical reaction mixture, 12-crown-4, 15-crown-5, and 18-crown-6 were formed in 15%, 5% and 4% yields respectively. Dioxane constituted 40% of the product mixture and the remainder was less than 3% each of identified components below the cyclododecamer. ... 

A new crosslinkable polymer was synthesized by the SBP-catalyzed polymerization of cardanol. When HRP was used as catalyst for the cardanol polymerization, the reaction took place in the presence of a redox mediator (phe-nothiazine derivative) to give the polymer. Fe-salen efficiently catalyzed the polymerization of cardanol in organic solvents (Scheme 29). " The polymerization proceeded in 1,4-dioxane to give the soluble polymer with molecular weight of several thousands in good yields. The curing of the polymer took place in the presence of cobalt naphthenate catalyst at room temperature or thermal treatment (150°C for 30 min) to form yellowish transparent films ( artificial urushi ... 

In many polymerizations cyclic material is produced by a concurrent backbiting reaction as linear polymer is formed. For example dioxan Is formed In the cationic polymerization of ethylene oxide to polyoxyethylene, and polyoxyethylene can be degraded to... 

Chojnowski and co-workers have studied the polymerization of octamethyltetrasila-l,4-dioxane, a monomer more basic than cyclosiloxanes, which is capable of forming more stable oxonium ions, and thus being a useful model to study the role of silyloxonium ions.150-152 In recent work, these authors used Olah s initiating system and observed the formation of oxonium ion and its transformation to the corresponding tertiary silyloxonium ion at the chain ends.153 The 29Si NMR spectroscopic data and theoretical calculations were consistent with the postulated mechanism. Stannett and co-workers studied an unconventional process of radiation-initiated polymerization of cyclic siloxanes and proposed a mechanism involving the intermediate formation of silicenium ions solvated by the siloxane... 

Phase separation controlled by diffusion exchange often results in a skin which is composed of a micellar assembly of nodules, as will be discussed below. When extremely hydrophobic polymers (e.g., modifled-PPO) are cast from dioxane into water (pg = p = p ) a dense polymer layer is formed at the solution s interface that somewhat resembles the type of layer formed by Interfacial polymerization. There is almost no inward contraction of the interfacial skin, and the coagulation process is controlled by diffusion through the dense, interfacial thin film. These result in an anisotropic membrane with a very fine "coral" structure (Figures 9 and 10). 

D.J. Worsfold In dioxane solution the polymer produced is from reaction of the ion pair almost entirely. As the NMR spectra are taken in concentrated solution, even in THF the structure observed is the ion pair and should correlate with the polymer structure formed in dioxane. In polymerizations in THF solution the polymer is quite largely formed from the reaction of the free ion chain end, and hence is less dependent on the ion pair structure. The NMR spectra were measured at -20 C because these compounds are not very stable, but as dioxane freezes at +10 C it was not possible to use it for the NMR solvent. 

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